Each of the atomic operations is guaranteed to be atomic across multiple
threads and in the presence of interrupts.
They can be used to implement reference counts or as building blocks for more
advanced synchronization primitives such as mutexes.

By default, a threads accesses to different memory locations might not be
performed in
program order,
that is, the order in which the accesses appear in the source code.
To optimize the programs execution, both the compiler and processor might
reorder the threads accesses.
However, both ensure that their reordering of the accesses is not visible to
the thread.
Otherwise, the traditional memory model that is expected by single-threaded
programs would be violated.
Nonetheless, other threads in a multithreaded program, such as the
.Fx
kernel, might observe the reordering.
Moreover, in some cases, such as the implementation of synchronization between
threads, arbitrary reordering might result in the incorrect execution of the
program.
To constrain the reordering that both the compiler and processor might perform
on a threads accesses, the thread should use atomic operations with
acquire
and
release
semantics.

Most of the atomic operations on memory have three variants.
The first variant performs the operation without imposing any ordering
constraints on memory accesses to other locations.
The second variant has acquire semantics, and the third variant has release
semantics.
In effect, operations with acquire and release semantics establish one-way
barriers to reordering.

When an atomic operation has acquire semantics, the effects of the operation
must have completed before any subsequent load or store (by program order) is
performed.
Conversely, acquire semantics do not require that prior loads or stores have
completed before the atomic operation is performed.
To denote acquire semantics, the suffix
"_acq"
is inserted into the function name immediately prior to the
"_<type>"
suffix.
For example, to subtract two integers ensuring that subsequent loads and
stores happen after the subtraction is performed, use
atomic_subtract_acq_int.

When an atomic operation has release semantics, the effects of all prior
loads or stores (by program order) must have completed before the operation
is performed.
Conversely, release semantics do not require that the effects of the
atomic operation must have completed before any subsequent load or store is
performed.
To denote release semantics, the suffix
"_rel"
is inserted into the function name immediately prior to the
"_<type>"
suffix.
For example, to add two long integers ensuring that all prior loads and
stores happen before the addition, use
atomic_add_rel_long.

The one-way barriers provided by acquire and release operations allow the
implementations of common synchronization primitives to express their
ordering requirements without also imposing unnecessary ordering.
For example, for a critical section guarded by a mutex, an acquire operation
when the mutex is locked and a release operation when the mutex is unlocked
will prevent any loads or stores from moving outside of the critical
section.
However, they will not prevent the compiler or processor from moving loads
or stores into the critical section, which does not violate the semantics of
a mutex.

In multiprocessor systems, the atomicity of the atomic operations on memory
depends on support for cache coherence in the underlying architecture.
In general, cache coherence on the default memory type,
VM_MEMATTR_DEFAULT,
is guaranteed by all architectures that are supported by
.Fx .
For example, cache coherence is guaranteed on write-back memory by the
amd64
and
i386
architectures.
However, on some architectures, cache coherence might not be enabled on all
memory types.
To determine if cache coherence is enabled for a non-default memory type,
consult the architectures documentation.
On the
ia64
architecture, coherency is only guaranteed for pages that are configured to
using a caching policy of either uncached or write back.

The
atomic_cmpset
function returns the result of the compare operation.
The
atomic_fetchadd,
atomic_load,
atomic_readandclear,
and
atomic_swap
functions return the value at the specified address.
The
atomic_testandset
function returns the result of the test operation.

This example uses the
atomic_cmpset_acq_ptr
and
atomic_set_ptr
functions to obtain a sleep mutex and handle recursion.
Since the
mtx_lock
member of a
.Vt struct mtx
is a pointer, the
"ptr"
type is used.

The
atomic_add,
atomic_clear,
atomic_set,
and
atomic_subtract
operations were first introduced in
.Fx 3.0 .
This first set only supported the types
"char",
"short",
"int",
and
"long".
The
atomic_cmpset,
atomic_load,
atomic_readandclear,
and
atomic_store
operations were added in
.Fx 5.0 .
The types
"8",
"16",
"32",
"64",
and
"ptr"
and all of the acquire and release variants
were added in
.Fx 5.0
as well.
The
atomic_fetchadd
operations were added in
.Fx 6.0 .
The
atomic_swap
and
atomic_testandset
operations were added in
.Fx 10.0 .